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ILC BDS Alignment and Tuning Studies. Glen White SLAC/QMUL 8 November 2005. Progress report and ongoing plans for BDS alignment and tuning strategy. BDS Alignment and Tuning Simulations. Using most recent (pre-Snowmass) 20mrad BDS deck plus extraction line.

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Ilc bds alignment and tuning studies

ILC BDS Alignment and Tuning Studies

Glen White


8 November 2005

  • Progress report and ongoing plans for BDS alignment and tuning strategy.

Bds alignment and tuning simulations
BDS Alignment and Tuning Simulations

  • Using most recent (pre-Snowmass) 20mrad BDS deck plus extraction line.

  • Start with expected post-survey magnet and BPM alignment tolerances, magnet errors and BPM resolutions.

  • Simulate BPM-Magnet alignment using Quad-shunting technique and fits to higher-order magnet moves (Sexts, Octs).

  • Steer/move to BPM readings with measured alignment.

  • Generate orthogonal knobs for correction of IP aberrations using Sextupole movers.

  • Simulation tool used: Lucretia.

Initial parameter assumptions
Initial Parameter Assumptions

  • All magnets have an associated BPM and x, y and roll movers.

  • Quads have x- and y- corrector dipoles.

  • Magnet RMS mis-alignment: 200um.

  • Assume initial BPM-magnet centre alignment of 30um.

  • Magnet rotation: 300urad.

  • RMS relative magnetic strength error: 0.1%.

  • Magnet mover resolution (x & y): 50nm.

  • BPM resolution: 1um.

  • Initially, use TESLA/USC bunch parameters (ideal gaussian for now) with 0.1 % uncorl. E spread.

  • Track 2000 macro-particles per bunch, 100 random seeds.

Alignment tuning strategy
Alignment & Tuning Strategy

  • Switch off Sextupoles & Octupoles.

  • Apply 1-1 Steering algorithm to align beam to measured BPM centres in Quads.

  • Use nulling Quad-shunting technique to get BPM-Quad alignments.

  • Use Quad movers to put Quads in a straight line in x and y with beam going through Quad field centres.

  • Get BPM- Sextupole alignment with movers and use a fit to downstream BPM responses.

  • Switch on Sextupoles and use Sextupole multi-knobs to tune IP waist, dispersion and coupling.

  • Perform BPM-Octopole alignment in a similar fashion to (5).

Bpm quad alignment
BPM-Quad Alignment

  • Nulling Quad Shunting technique:

    • To get BPM-Quad offsets, use downstream 10 Quad BPMs for each Quad being aligned (using ext. line BPMs for last few Quads).

    • Switching on & off Quad’s power, use change in downstream BPM readouts to get Quad offset.

    • Move Quad and repeat until detect zero-crossing.

    • For offset measurement, use weighted-fit to downstream BPM readings based on ideal model transfer functions:

Quadrupole alignment results
Quadrupole Alignment Results

  • Left: BPM-Quad alignments (RMS mis-alignment from 100 seeds).

  • Right: RMS Quad position post Quad mover alignment routine.

    • End points not fixed -> how to achieve initial beam collisions?

Sextupole octopole alignment
Sextupole, Octopole Alignment

  • Use x-, y-movers on higher-order magnets and fit 2nd, 3rd order polynomials to downstream BPM responses (for Sext, Oct respectively).

    • Alignment is where 2nd, 3rd derivative is 0 from fits.

Beam orbit post alignment
Beam Orbit Post-Alignment

  • Beam orbit as measured in Quad BPMs after magnet alignment

Generating ip tuning knobs
Generating IP Tuning Knobs

  • Use x- and y-moves of 5 Sextupole magnets to generate IP x- and y-dispersion and waist tuning knobs and (4) x-y coupling knobs.

  • Generate response matrix to map Sextupole movements to IP parameters.

  • Use SVD matrix inversion to get tuning knobs.

  • With full errors applied, found knobs to be not so orthogonal – applied iterative tuning procedure based on IP beam spot sizes.

  • IP RMS pre-knob tuning errors are (for 100 seeds):

    • Waist (x,y) ~ 0.5mm

    • Dispersion (x) ~ 0.5 mm (y) ~ 6.5 um

    • X-Y Coupling: (<xy>) ~ 0.76 (<xy’>) ~ 0.34

    • (<x’y>) ~ 0.82 (<x’y’>) ~ 0.31

Application of ip multi knobs
Application of IP Multi-Knobs

  • Start with just aligned Sextupoles on (no Octs).

  • Zero x- and y- IP dispersion based on IP dispersion measurements.

  • Iteratively tune all other knobs based on IP spot sizes (~lumi).

  • Align and switch on Octupoles.

  • Sweep each Oct in x and y, minimizing IP spots to reduce non-linear aberrations introduced (need to develop tuning knobs for this later).

  • Re-apply Sextupole tuning knobs.

Application of ip multi knobs1
Application of IP Multi-Knobs


Steer Quads

Align Quads

Align Sexts



Oct Align + MK

Post tuning ip beam sizes
Post-Tuning IP Beam Sizes

  • Final IP params (100 seeds):

    • RMS Dispersion (x/y): 0.39 mm / 1.5 um

    • RMS Coupling Terms: <xy> 0.14 <x’y> 0.01 <xy’> 0.34 <x’y’> 0.33

Future bds alignment work
Future BDS Alignment Work

  • Improve IP multi-knobs

    • Include non-linear terms for Sextupole knobs.

    • Use Skew Quads in addition for coupling correction.

  • Simulate 2 beams- tune on luminosity (pair signals).

  • Include steering routine to get initial beam collisions.

  • Include LINAC to get real bunch shapes.

  • Include GM.

  • Integrated time-evolved simulation with initial tuning + pulse-pulse FB + intra-bunch FB.

    • Provides information on how often re-tuning necessary and most detailed luminosity performance estimate.